Support specifying ssl_password_file for nginx

What does this MR do?

Support specifying ssl_password_file in nginx conf

Testing

1. Deploy GitLab instance using HTTP
2. Create self-signed certificate for the GitLab FQDN with a private (encrypted) key
3. Edit gitlab.rb to change GitLab instance to use HTTPS with this self-signed certificate and run sudo gitlab-ctl reconfigure
4. Try accessing GitLab instance and see it fails
5. Create /etc/gitlab/ssl/nginx_password_file with the passphrase to the SSL key
6. Edit /etc/gitlab/gitlab/rb and append nginx['ssl_password_file'] = '/etc/gitlab/ssl/nginx_password_file' to it and run sudo gitlab-ctl reconfigure
7. Run sudo gitlab-ctl tail nginx and see that nginx is now running fine.
8. Try accessing GitLab instance from another machine and see certificate works

Related issues

Closes #4181 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened